Conventionally, various measuring apparatuses have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, known is a gas sensor having an electrochemical pumping cell structured by forming a Pt electrode and a Rh electrode on an oxygen-ion conductive solid electrolyte layer, such as a zirconia (ZrO2) layer. In this gas sensor, an electrode protecting layer constituted by a porous body is formed so as to cover a measuring electrode. The electrode protecting layer serves to limit the amount of NOx flowing into the measuring electrode, and also to protect the measuring electrode.
For example, in a known gas sensor, the electrode protecting layer is formed with a sufficient thickness even near end portions of the measuring electrode to thereby effectively prevent occurrence of cracking in the electrode protecting layer and peeling-off of the electrode protecting layer, which may otherwise be caused by a stress generated by repetitive oxidation/reduction of an electrode metal, or the like (for example, see Japanese Patent Application Laid-Open No. 2006-284223).
However, there is the following problem. When the gas sensor as disclosed in Japanese Patent Application Laid-Open No. 2006-284223 is mounted in an exhaust system of an internal combustion engine such as an automobile engine and the internal combustion engine is driven, harmful substances (Mg, Na, Ca, and the like) contained in the exhaust gas may clog the electrode protective layer constituted by the porous body. Such clogging of the electrode protective layer makes it difficult that a measurement gas reaches the measuring electrode, so that the sensitivity of the gas sensor to a measurement gas component such as NOx, in other words, a measurement accuracy, gradually deteriorates along with the use of the gas sensor.
Additionally, in the gas sensor as disclosed in Japanese Patent Application Laid-Open No. 2006-284223, occurrence of cracking or peeling-off of the electrode protective layer is effectively prevented by devising the structure of the electrode protective layer, but a stress is still applied to the electrode protective layer because of repetitive oxidation/reduction of the electrode metal, or the like. That is, the above-described gas sensor involves a problem that the possibility of occurrence of cracking or peeling-off of the electrode protective layer increases along with the use, and the measurement accuracy deteriorates due to the occurrence of cracking, peeling-off, or the like, of the electrode protective layer.